In the measurement of the caliper of paper, caliper gauges are of both the contacting and non-contacting types. Contacting gauges are believed to be more reliable (or accurate) but because they bear against the sheet material they may cause tearing or other unwanted effects. In addition to the fact that the sheet material (such as newsprint) is fluttering and shifting, the paper material may contain unwanted bumps or other discontinuities such as holes. This is especially true in the case of recycled-type newsprint. And where the paper sheet may be moving at a speed of as much as 20 meters per second, a small tear may propagate across the entire sheet. This may also be the case where later in the process the sheet is being coated and there again the sheet if it has a slight tear in it is susceptible to a rip across its entire width.
While a non-contacting gauge which, for example, might utilize air jets to space the gauge from the surface avoids the foregoing problem somewhat, it is less reliable and in general less accurate.